The present invention relates generally to semiconductor devices and, more particularly, to a method for determining the concentration of nitrogen in a film of nitrided oxide material formed over a semiconductor wafer during fabrication of a semiconductor device.
Spurred by the demand for smaller and faster devices, the semiconductor industry has shifted to the use of thinner gate oxides to increase the speed of semiconductor devices. Thinner gate oxides are problematic, however, because they are susceptible to hot electrons and to the diffusion of materials through the oxide into the channel area of the gate. In 0.25 .mu.m and 0.20 .mu.m generation semiconductor devices, e.g., CMOS transistors, nitrogen has been added to thin silicon dioxide gate oxides having a thickness of, e.g., about 50 angstroms, to improve the quality of these devices.
FIG. 1 is a schematic cross-section of a conventional MOS transistor in which source/drain regions 102 are formed in semiconductor substrate 100, which is typically silicon. Polysilicon gate 104, which is separated from substrate 100 by gate oxide 106, extends over the channel region of the MOS transistor. When nitrogen is added to gate oxide 106, which is typically silicon dioxide, the nitrogen incorporates at the silicon/silicon dioxide interface between substrate 100 and gate oxide 106. The formation of Si--N bonds at the interface increases the strength of the interface because Si--N bonds are stronger than Si--O bonds. This increased strength due to the presence of nitrogen increases the resistance of the gate oxide to hot electrons and thereby improves the hot carrier lifetime of the gate oxide. The presence of nitrogen also reduces the diffusion of, e.g., boron, through the oxide into the channel area of the gate.
In commercial production in a fab environment, nitrided gate oxides may be formed by growing a thin film of silicon dioxide having a thickness of, e.g., about 40 angstroms in the presence of NO or other suitable nitric species, e.g., N.sub.2, NH.sub.3, and N.sub.2 O. At present, quantitative characterization of the nitrogen at the interface is limited to secondary ion mass spectroscopy (SIMS) because of the small number of nitrogen atoms in the thin gate oxide. Determining the nitrogen concentration in nitrided silicon dioxide films using the SIMS technique is undesirable, however, for at least four reasons. First, the SIMS technique is expensive (about $400 per wafer) and requires a long measurement time per wafer. Second, the SIMS technique is destructive, i.e., each wafer that is tested is destroyed. Third, the area of a wafer that can be tested by the SIMS technique is very small. Fourth, it is impractical to use the SIMS technique in a fab environment.
In light of the problems associated with the SIMS technique, the nitrogen concentration in nitrided gate oxides being commercially produced in fab environments is not being monitored frequently. Before high-volume production begins, SIMS testing is performed to determine the nitrogen concentration in thin films formed using an initial set of process parameters and any necessary adjustments are made. Once SIMS testing confirms that the established process parameters yield the desired nitrogen concentration, the process is used in high-volume production and the nitrogen concentration is assumed to remain at the desired level. Thereafter, the SIMS testing is repeated only about every six months to confirm that the nitrogen concentration is still at the desired level.
In practice, however, changed circumstances may cause the nitrogen concentration to vary unexpectedly. For example, if the nitrogen source runs out, then one or more lots of wafers may be produced with gate oxides that do not include nitrogen. As discussed above, such gate oxides are undesirable because they have a relatively short hot carrier lifetime and materials, e.g., boron, can diffuse therethrough into the channel area of the gate. On the other hand, if something happens that causes too much nitrogen to be introduced into the furnace, then the excess nitrogen may spread into the channel and degrade the conduction therein. In either scenario, the potential exists for thousands of dollars of wafers to be ruined before the problem is discovered.
In view of the foregoing, there is a need for a method that enables the concentration of nitrogen in a film of nitrided oxide material to be monitored quickly and inexpensively in a fab environment.